1. Field of the Invention
This invention relates to a developer carrying member used in a developing apparatus for carrying out development by the use of a developer, used in electrophotography, electrostatic recording, magnetic recording, etc., and also relates to an apparatus unit and an image forming apparatus which employ such a developer carrying member.
More particularly, this invention relates to a developer carrying member used in a developing apparatus of the system in which a dry-process (powdery) developer is fed to and carried on the surface of a developer carrying member, and the developer thus carried thereon is layer-regulated into a thin layer by means of a layer-thickness regulating member (layer-regulating member) and then transported to a developing zone where the developer carrying member faces the surface of a latent image bearing member, to make the latent image render visible; and also relates to an apparatus unit and an image forming apparatus which employ such a developer carrying ember.
2. Related Background Art
A number of methods are conventionally known as electrophotography. In general, copies are obtained by forming an electrostatic latent image on an electrostatic latent image bearing member (photosensitive member) by utilizing a photoconductive material and by various means, subsequently developing the electrostatic latent image by the use of a toner (developer) to make it visible to form a toner image, transferring the toner image to a transfer medium such as paper as occasion calls, and then fixing the toner image to the transfer medium by heating, pressing or the like.
In recent years, in addition to conventional copying machines, instruments making use of electrophotography are used in various apparatus such as printers and facsimile machines. Especially in printers and facsimile machines, their copying apparatus part must be made smaller, and hence developing apparatus employing one-component developers are often used.
One-component developing systems making use of one-component developers are methods in which electric charges having a polarity reverse to that of electric charges of the electrostatic latent image formed on a photosensitive drum and to the development standard potential are imparted to toner particles by the friction between the toner particles themselves and the friction between a developing sleeve as the developer carrying member and the toner particles, the toner thus charged is very thinly coated on the developing sleeve and then transported to the developing zone where the photosensitive drum faces the developing sleeve, and in the developing zone the toner is caused to adhere to the surface of the photosensitive drum to carry out development to make the electrostatic latent image visible as a toner image.
Such one-component development systems require no carrier particles such as glass beads or iron powder required in two-component development systems, and hence can make developing assemblies themselves small-sized and light-weight. Also, since in the two-component development systems the concentration of toner in developer must be kept constant, a device for detecting toner concentration so as to supply the toner in the desired quantity is required, resulting in an increase in size and weight of the developing assemblies. In the one-component development system, such a device is not required, and hence the developing assemblies can be made small and light-weight as is preferable.
As printers, LED printers or LBP printers are prevailing in the recent market. As a trend of techniques, there is a tendency toward higher resolution. That is, those which hitherto have a resolution of 300 or 400 dpi are being replaced by those having a resolution of 600, 800 or 1,200 dpi. Accordingly, with such a trend, the developing systems are now required to achieve a degree of minuteness.
Copying machines have also made progress to have high functions, and hence the trend is toward digital systems. In this trend, chiefly employed is a method in which electrostatic latent images are formed by using a laser. Hence, the copying machines also trend toward a high resolution and, like the printers, it has been sought to provide a developing system with high resolution and high minuteness. Accordingly, toners having small particle diameters are proposed in Japanese Patent Application Laid-Open Nos. 1-112253 and 2-284158, and toners are being made to have smaller particle diameters.
As the developer carrying member used in the development of the above system, a member is used which is produced by molding, e.g., a metal, an alloy or compound thereof into a cylinder and treating its surface by electrolysis, blasting or filing so as to have a stated surface roughness. In such an instance, however, in the developer layer regulated by the regulating member into a thin layer and formed on the developer carrying member surface, the developer present on the developer carrying member surface and in the vicinity thereof comes to have a very high electric charge, so that it is strongly attracted to the developer carrying member surface by the action of mirror force. This makes the toner particles have no opportunity of their friction with the carrying member, and hence the developer comes to have no preferable electric charges (a phenomenon of what is called "charge-up"). Under such a condition, satisfactory development and transfer cannot be carried out, resulting in images with much uneven image density and many black spots around line images.
In order to prevent the occurrence of such a developer having excessive electric charges and to prevent strong adhesion of the developer, as disclosed in Japanese Patent Application Laid-Open No. 1-277265, a method is proposed in which a coating film of a resin with a conductive material such as carbon black or graphite powder or a solid lubricant dispersed therein is formed on the developer carrying member.
In recent years, it has again become required to save energy consumed in copying machines and LBP (laser beam printer) main bodies. This has brought with it studies energetically made on how to fix at low temperature the developer used, in order to save energy necessary for the fixing. Under the influence of such low-temperature fixing, there is an increase in developers which tend to cause their melt-adhesion to developing sleeves. Thus, giving a preference to the fixing performance may make it difficult to well ensure the developing performance.
In such a trend toward making toner particles finer and fixing temperature lower, it is needed to provide a method by which a sufficient, uniform and high electric charge can be imparted to the toner and also the mirror force can be prevented from acting between the toner and the sleeve.
In the trend toward higher image quality of electrophotography in recent years, there is a tendency that, in order to make image quality much higher, the developer is made to have smaller average particle diameter and also the developer is more strongly regulated in the constitution of developing assemblies so that the developer can be carried on the developer carrying member in a thiner layer. This brings about an increase in physical load against the developer and developer carrying member to more likely cause the above charge-up and also sleeve ghost images.
However, in the developer as stated above, made to have a smaller particle diameter, it is often attempted to increase the content of a magnetic material, to select materials that may collect not too much charge on the toner particle surfaces or to select an external additive having the ability to let charges leak, in order to prevent the developer from being irregularly coated on the developer carrying member. In such instances, the charging of toner strongly tends to rise slowly. In addition, there is a tendency that, as a way of realizing the low-temperature fixing stated above, Tg (glass transition point) of the developer is set a little lower, a low-molecular-weight component is added to binder resin in a little larger quantity, or a low-melting component such as wax is added in a little larger quantity. Use of such materials may make it difficult for the toner to be well charged, often resulting in a decrease in charge quantity to cause a lowering of developing performance. Hence, if only the technique disclosed in the aforesaid Japanese Patent Application Laid-Open No. 1-277265 is relied on, a sufficient charge can not be imparted to the developer, bringing about unsatisfactory results.
Accordingly, it is necessary for the developer carrying member to be more improved in its charge-providing performance to the developer to more likely to prevent the phenomenon of charge-up. Moreover, under the circumstances that the developer layer thickness is more strongly regulated and cartridges are more frequently used or made to have a larger capacity in recent years, it is required for the developer carrying member to have a sufficient wear resistance and a uniformity in its resin coat layer.
Japanese Patent Application Laid-Open No. 56-146167 discloses that, in a one-component developing system, the surface of a member that imparts triboelectricity to toner, i.e., of a toner carrying member contains an organic polymer containing a specific nitrogen-containing group, and its Examples disclose, e.g., a copolymer of dimethylaminoethyl methacrylate with a styrene monomer or a copolymer of p-dimethylaminostyrene monomer with a methyl methacrylate monomer.
However, in the above Japanese Patent Application Laid-open No. 56-146167, the toner carrying member specifically prepared in the Examples are obtained by forming a coating film on the sleeve surface by the use of a solution prepared by adding to a solvent the organic polymer containing a nitrogen-containing group, and hence the coating film has insulating properties. Thus, it is neither disclosed nor suggested how to improve the mechanical strength of coating film, the melt-adhesion resistance of toner against coating film and the dispersibility of conductive fine powder in coating film when the conductive fine powder is added in the coating film of the developer carrying member so as to decrease electrical resistance of the coating film, for the purposes of improving triboelectric charging performance and also making it stable.
In addition, in the above Japanese Patent Application Laid-Open No. 56-146167, the layer thickness of the toner layer formed on the toner carrying member is regulated by the action of a magnetic binding force acting between an iron doctor blade provided in proximity to the toner carrying member surface and a multi-polar permanent magnet provided inside the toner carrying member. Thus, there is room for further improvement in the stability of triboelectric charging performance of toner to environmental variations.
Techniques in which a elastic blade is brought into touch with the toner layer on the toner carrying member in order to make the triboelectric charging performance stable to variations of external environmental conditions are disclosed in Japanese Patent Application Laid-Open Nos. 54-43038 and 58-116559.